Surface Plasmon Resonance (SPR) occurs when surface plasmon waves, which are collective oscillations of electrons in a metal, are excited at a metal/dielectric interface. Light is directed at, and reflected from, the side of the metal film surface not in contact with the sample. SPR causes a reduction in the reflected light intensity at a specific combination of angle and wavelength. Biomolecular binding events lead to a change in the refractive index and thickness of an ultra-thin organic (dielectric) layer on the metal film, which changes the SPR resonance conditions resulting in an extremely high sensitivity response. SPR methodology does not require a time-consuming labeling step, and reaction kinetics constant can be routinely obtained within minutes.
Commercially available SPR-based devices use light intensity (angular or spectral position of SPR minimum) as the information source. The detection limit of conventional SPR devices is of the order of 10−5 or higher in terms of refractive index change (5×10−6 in advanced configurations with acoustic-optical modulators). This corresponds to a minimum detection limit of about 1 pg per mm−2 of biomaterial accumulating at the biosensor surface. In general, this sensitivity is sufficient to study molecular interactions, for example antibody-antigen, protein-DNA, DNA-DNA, receptor-ligand, etc.
The SPR interferometry approach, in which information on phase is extracted optically from spatial interference pattern formed by interfering signal and reference (non-affected by SPR) beams, is a common approach. This approach gives high sensitivity and possibility of lateral resolution, but has disadvantages such as narrow dynamic range, low noise immunity, and a complicated image treatment. Alternatively, using an polarimetric approach, information on phase can be obtained from the analysis of the ellipse of polarization. In particular, a polarimetric scheme, in which phase information was studied with the help of a rotating analyzer and electro-optic modulators, has been reported. Such approach improved noise immunity and enabled to apply electronic processing for signal filtering. However, the dynamic range of measurements remained narrow, and the measurement procedure was complicated.
Based on the foregoing, there is an ongoing need to develop SPR methodology with high sensitivity and wide dynamic range. SPR methodology with these qualities would be very useful in the analysis of biological and chemical samples, particularly in the detection of small molecular weight analytes (such as molecules with a mass of less than 1000 Daltons) and other biomolecules.